The following invention relates to the polishing of radially expandable surgical stents which can be surgically implanted into a body lumen, such as an artery, and be radially expanded to support the lumen. More specifically, this invention relates to fixtures used for supporting a radially expandable surgical stent while an abrasive media is flowed over surfaces of the stent to polish the stent and provide an inner surface of the stent with a streamlined contour, and methods for using such fixtures while polishing surgical stents.
Surgical stents have long been known which can be surgically implanted into a body lumen, such as an artery, to reinforce, support, repair or otherwise enhance the performance of the lumen. For instance, in cardiovascular surgery it is often desirable to place a stent in the coronary artery at a location where the artery is damaged or is susceptible to collapse. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery. One form of stent which is particularly desirable for implantation in arteries and other body lumens is a tubular stent which is formed as a complete tubular cylinder and can be radially expanded from a first smaller diameter to a second larger diameter. Such radially expandable stents can be inserted into the artery by being located on a catheter and fed internally through the arterial pathways of the patient until the unexpanded stent is located where desired. The catheter is fitted with a balloon or other expansion mechanism which exerts a radial pressure outward on the stent, causing the stent to expand radially to a larger diameter. Such expandable stents exhibit sufficient rigidity after being expanded that they will remain expanded after the catheter has been removed.
Radially expandable stents come in a variety of different configurations to provide optimal performance in various different particular circumstances. For instance, the patents to Lau (U.S. Pat. Nos. 5,514,154, 5,421,955, and 5,242,399), Baracci (U.S. Pat. No. 5,531,741), Gaterud (U.S. Pat. No. 5,522,882), Gianturco (U.S. Pat. Nos. 5,507,771 and 5,314,444), Termin (U.S. Pat. No. 5,496,277), Lane (U.S. Pat. No. 5,494,029), Maeda (U.S. Pat. No. 5,507,767), Marin (U.S. Pat. No. 5,443,477), Khosravi (U.S. Pat. No. 5,441,515), Jessen (U.S. Pat. No. 5,425,739), Hickle (U.S. Pat. No. 5,139,480), Schatz (U.S. Pat. No. 5,195,984), Fordenbacher (U.S. Pat. No. 5,549,662) and Wiktor (U.S. Pat. No. 5,133,732), each include some form of radially expandable stent for implantation into a body lumen.
Some problems which have been exhibited by prior art stents include that the inner and outer surfaces of the stents are not sufficiently streamlined or finely enough polished to prevent certain medical complications. For instance, thrombus, a phenomenon where a fibrous clot forms within cracks and other irregularities in the surface finish of an implanted object (such as a stent), is enhanced when the surfaces of the stent are not finely polished. Additionally, when the inner surface of the stent is substantially planar and has abrupt edges along borders thereof, turbulence is introduced into the blood. When a stent having such an abrupt edge is implanted into an artery, plaque and other deposits are provided with a site for collection and potential narrowing of the arteries and restriction of blood flow. This plaque buildup adjacent an implanted object (such as a stent) is referred to as xe2x80x9crestenosis.xe2x80x9d
While many prior art stents do exhibit somewhat polished surfaces, they are typically not sufficiently finely polished, especially on tubular stents having smaller diameters, to prevent restenosis and thrombus adjacent the stent after the stent is implanted into the artery. Such prior art stents also lack a streamlined contour to minimize disruption of bodily fluid flow through the lumen and to further discourage restenosis surrounding the stent.
A primary reason why prior art stents fail to exhibit sufficiently finely polished surfaces to avoid the drawbacks discussed above is the lack of a polishing process which can effectively provide the finely polished surface desired, especially on stents having smaller inner diameters. Stents are typically polished in one of two processes, either chemical etching or electropolishing. With chemical etching, chemicals are used which react chemically with the material forming the stent, causing the material forming the stent to be driven into solution. Chemicals are selected which have a strength sufficient to cause rough areas of the stent to be dissolved, but not so strong that smooth areas of the stent are detrimentally altered. Chemical etching, while somewhat effective in removing gross irregularities from the surfaces of the stent, fail to adequately provide the desired finely polished surface.
Electropolishing typically involves providing an electrolytic solution, placing the stent within the electrolytic solution, placing a cathode within the solution and not contacting the stent and coupling an anode to the stent. When an electric voltage is provided between the anode and the cathode, the stent is caused to lose portions of its outer surface when the elements forming the stent are driven into solution and carried to the cathode for deposition upon the cathode. In essence, such electrolytic polishing is the reverse of commonly used electrical plating processes with material from the surface of the stent being removed rather than added to the stent. The rougher surfaces of the stent are more readily driven into solution and hence removed from the surfaces of the stent, smoothing the surfaces of the stent somewhat.
Because the surfaces of the stent forming the inner diameter of the stent benefit from a high degree of polishing, one known technique is to form the cathode as a thin wire passing along a central axis of the stent entirely through the stent from one end to the other, but without physically contacting the stent. When a voltage is provided between the cathode wire passing along the central axis of the stent and the stent itself, the inner surfaces of the stent are provided with the greatest electric field density and hence are the surfaces which are most polished during this process. While typically more effective than chemical etching, electrolytic polishing also fails to provide a sufficiently finely polished stent to significantly discourage thrombus and restenosis adjacent surfaces of the stent.
Accordingly, a need exists for a method and apparatus for polishing surfaces of a radially expandable surgical stent, and particularly the surfaces forming the inner diameter of the stent, with a sufficient degree of polish to reduce or eliminate the occurrence of thrombus and restenosis when surgical stents are implanted within a body lumen.
The radially expandable surgical stent which is polished and streamlined by the method and apparatus of this invention exhibits an overall tubular cylindrical hollow seamless contour which can feature any of a variety of different arrangements for individual elements and segments forming the stent. The various different segments of the stent have a generally elongate, substantially constant cross-sectional contour which can either be oriented to extend axially, circumferentially, or some combination thereof, with each segment located between an inner diameter of the stent and an outer diameter of the stent. Each segment includes an outer surface coextensive with the outer diameter of the stent and an inner surface coextensive with the inner diameter of the stent. Each segment also includes lateral surfaces extending between the inner surface and the outer surface which can either be a leading surface on an upstream side of the segment, a trailing surface on a downstream side of the segment, or a lateral surface generally aligned axially with the stent.
The inner surface of each segment of the stent is extensively streamlined by the polishing method and apparatus of this invention to minimize disruption of bodily fluid flow through the body lumen. Specifically, the inner surface includes an inner leading edge and an inner trailing edge bordering the inner surface. Each inner edge is defined by an inner curve having a relatively large radius of curvature when compared to the radii of curvature exhibited by outer edges adjacent the outer surface of each stent segment. Because the inner edges have a large radius of curvature, they do not present any abrupt transition in flow for bodily fluids passing over the inner surface of the stent segment, particularly when the stent segment is aligned circumferentially with bodily fluid flow passing adjacent the inner surface from a leading inner edge to a trailing inner edge.
The surfaces of each stent segment are honed and polished to have a surface finish which is free from abrupt transitions and irregularities, such as prominences extending more than five micro inches above adjacent portions of the surrounding surface. Smooth flow of blood or other bodily fluids over the surfaces of the stent can thus be preserved and a risk of medical complications such as restenosis and thrombus can be minimized.
The polishing apparatus of this invention includes a fixture which rigidly supports at least one radially expandable surgical stent within a cylindrical chamber in the fixture. A bore passes through the fixture and leads both into the cylindrical chamber and out of the cylindrical chamber. A source of fluid abrasive media is placed adjacent the fixture in an orientation which allows the fluid abrasive media to pass through the bores and into the cylindrical chamber. The cylindrical chamber has a diameter similar to the outer diameter of the stent so that the fluid abrasive media is forced to pass only through the interior of the stent and adjacent the surfaces forming the inner diameter of the stent. As the fluid abrasive media passes through the cylindrical chamber and adjacent the surfaces forming the inner diameter of the stent, the surfaces forming the inner diameter of the stent are polished to a level of smoothness determined by the particle size of the abrasive media, the amount of time which the abrasive media flows past the surfaces of the stent and other factors known in the honing arts.
When it is desired that the outer diameter of the stent be polished, a stent exterior polishing fixture is provided having a cylindrical recess located therein with slanted bores leading from a top and bottom of the fixture to the cylindrical recess. The cylindrical recess has a diameter greater than the diameter of the outer diameter of the stent. A shaft is located within the cylindrical recess with a central axis of the shaft aligned with a central axis of the cylindrical recess. The shaft has a diameter similar to the inner diameter of the stent. The stent is placed on the shaft and within the cylindrical recess so that abrasive media flowing through the slanted bores and into the cylindrical recess are precluded from flowing adjacent the surfaces forming the inner diameter of the stent, but rather flow adjacent surfaces forming the outer diameter of the stent for polishing of the outer diameter of the stent.
In utilizing the various fixtures for supporting the stent during this polishing process, the stent is preferably initially provided with, radially-non-expandable cylindrical support ends adjacent each end of the stent. These cylindrical support ends are located along with the stent within the cylindrical chamber or cylindrical recess of one of the fixtures and provide additional support for the stent during the polishing process. The support ends prevent collapse of the stent and excessive polishing of ends of the stent during the polishing process.
The polishing process can be additionally facilitated by ultrasonically vibrating the abrasive media and elevating the pressure of the abrasive media as it flows through the fixture and adjacent surfaces of the stent. If it is desired that the stent be provided with a streamlined contour which is not biased in any one direction, the stent can be removed and reoriented within the fixture for polishing in a reverse direction or the fixture can be disconnected from the source of abrasive media, rotated 180xc2x0 and recoupled to the source of abrasive media for polishing in a reverse direction. Once the polishing process is completed, the cylindrical support ends are removed from the stent. The stent is then ready for implantation within a body lumen with such finely polished surfaces that restenosis and thrombus are minimized.
Accordingly, a primary object of the present invention is to provide a method for polishing surfaces of a radially expandable surgical stent which includes flowing a fluid abrasive media adjacent surfaces of the stent to be polished until the stent exhibits a desired finish.
Another object of the present invention is to provide a method for streamlining surfaces of a radially expandable surgical stent by flowing fluid abrasive media adjacent surfaces of the stent to be streamlined.
Another object of the present invention is to provide a method for polishing a radially expandable surgical stent which can polish multiple stents simultaneously.
Another object of the present invention is to provide a fixture for a radially expandable surgical stent polishing process which holds and supports the stent while fluid abrasive media is flowed adjacent surfaces of the stent and which can be easily loaded and unloaded with stents to be polished.
Another object of the present invention is to provide a fixture for a stent polishing process which restricts fluid abrasive media flow to the surfaces forming the inner diameter of the stent.
Another object of the present invention is to provide a fixture for a stent polishing process which restricts fluid abrasive media flow to the surfaces forming the outer diameter of the stent.
Another object of the present invention is to provide a stent polishing fixture which can be readily attached to honing equipment which uses elevated pressure fluid abrasive media and ultrasonic vibration of the fluid abrasive media and directs the fluid abrasive media through the fixture.
Another object of the present invention is to provide a surgical stent which minimizes medical complications such as restenosis and thrombus adjacent the stent.
Another object of the present invention is to provide a radially expandable surgical stent which has a finish smoothness which minimizes medical complications such as restenosis and thrombus adjacent the stent when the stent is implanted within an artery or other body lumen.
Another object of the present invention is to provide a surgical stent which can support a body lumen while minimizing disruption of flow of bodily fluids through the lumen.
Another object of the present invention is to provide a surgical stent which is reversible and can be implanted in two distinct orientations rotated 180xc2x0 from each other without altering performance of the surgical stent.
Another object of the present invention is to provide a surgical stent which features an inner surface which has edges with greater radii of curvature than radii of curvature of outer edges bordering an outer surface of segments of the stent, such that disruption to blood flow within a body lumen in which the stent is implanted is minimized and the outer surface of the stent is securely held adjacent a wall of the lumen.
Other further objects of the present invention will become apparent from a careful reading of the included description and claims and from a review of the drawing figures.